UNIST Develops Ultra-Compact Optical Device
Independent Control of Light Wavelength by Angle, Intensity by Voltage
Applications in Unhackable Quantum Communication and Precision Bio-Sensors
Research Published in Advanced Science
A chip the size of a grain of sand has been developed that can independently control both the wavelength and intensity (brightness) of light without interference.
This breakthrough paves the way for the creation of devices such as 'quantum entangled light sources,' which require real-time control of wavelength and intensity, as well as 'optical signal processing devices,' where miniaturization is essential.
The research team led by Professor Lee Jongwon from the Department of Electrical and Electronic Engineering at UNIST announced on December 16 that they have developed, for the first time in the world, a new 'metasurface' device capable of independently controlling the 'intensity' and 'wavelength' of light.
Research team (from left) Professor Lee Jongwon, Researcher Kim Jaesung (first author), Researcher Jung Hyeongju, Researcher Lee Sungjin. Provided by UNIST
A metasurface is an ultra-fine artificial device that arranges nanostructures much smaller than the wavelength of light on a surface, thereby manipulating the optical properties of light in ways not found in nature. It can replace bulky materials used in commercial optical modulation technologies, enabling the miniaturization of various devices, and can also create optical phenomena that are impossible with conventional optical modulation methods.
The metasurface developed by the research team controls a special optical phenomenon known as 'second harmonic generation (SHG).' Second harmonic generation is a technique that doubles the energy of input light (fundamental wavelength) and outputs it as new light with half the wavelength (second harmonic). For example, when infrared light is input, it is converted into light with a different wavelength, which can be used in sensors that detect trace amounts of biomolecules or in the development of quantum communication technologies that are immune to eavesdropping.
However, until now, this technology had the drawback that the wavelength and intensity of light were intricately linked. If one tried to increase the conversion efficiency to boost intensity, the range over which the wavelength could be controlled would narrow. Conversely, expanding the wavelength control range would result in a significant drop in efficiency, creating a trade-off.
The research team resolved this issue by adopting a device design strategy that separates the process of light manipulation within the metasurface into an 'entry' and an 'exit.' The process where light enters the chip and energy is accumulated (generation) and the process where the transformed light exits (emission) are handled by different control mechanisms. The team named this the 'local-to-nonlocal' approach.
A conceptual diagram showing the principle by which metasurface devices control the color (wavelength) and brightness (intensity) of light.
This metasurface chip, designed in this way, features two independent control mechanisms. By adjusting the voltage applied to the chip, the 'intensity' of light can be independently changed without altering the wavelength. Conversely, by slightly changing the angle at which light enters the chip, the 'wavelength' of light can be varied while the intensity remains constant. Thus, the properties of light can be perfectly separated and controlled without interference.
In actual experiments, when the research team adjusted the angle of incidence of the light, the output wavelength changed continuously. When they fixed a specific wavelength and only altered the electrical signal, the wavelength remained unchanged while only the intensity varied.
Professor Lee Jongwon explained, "Whereas previous research relied solely on one approach-either confining light (local mode) or allowing it to flow (nonlocal mode)-this technology combines both methods, making device design much more flexible and resolving the longstanding dilemma between efficiency and controllability."
He added, "This will contribute to the realization of next-generation active quantum light source technologies, such as real-time control of quantum information and the ability to freely tune the wavelength spectrum of 'entangled photons,' which are key to quantum communication."
This research was published in Advanced Science on November 29 and was supported by the Institute of Information & Communications Technology Planning & Evaluation and the National Research Foundation of Korea.
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